FIG. 1 (prior art) depicts a conventional Reconfigurable Optical Add Drop Multiplexer (ROADM) 100 suitable for processing a Wavelength Division Multiplexed (WDM) signal 105 having N wavelength channels. ROADM 100 receives, as input, WDM signal 105 carried on waveguide 110. WDM signal 105 is received by port 115 of optical circulator 120. The operation of optical circulator 120, a device known in the art, is such that an optical signal, such as WDM signal 105, is delivered to the next port along circulation direction 125, which is port 130. From port 130, signal 105 is launched along waveguide 135 towards a device suitable for separating or demultiplexing WDM signal 105 into its constituent spectral components 1051–105N. A waveguide grating router (WGR) 140, such as an Arrayed Waveguide Grating (AWG) or an array of thin film filters or Fiber Bragg Gratings, demultiplexes WDM signal 105 into its N spectral components.
The N spectral components of WDM signal 105 appear on the outputs of WGR 140 and are carried by waveguides 1421–142N to N 1×1 switches S11––S1N. These switches either reflect or pass light, as directed by control signals (not shown). In the reflective state, a reflective device, such as a metal or dielectric mirror, is positioned in the optical path of the spectral component traveling through a switch. For example, switch S12 is shown in a substantially reflective state, in which case a reflective device 145 causes substantially all optical energy of spectral component 1052 to be reflected back along waveguide 1422 to WGR 140. Spectral component 1052 passes through WGR 140 and emerges on waveguide 135, and is delivered to port 130 of optical circulator 120. Optical circulator 125 advances the spectral component to port 150 for “drop.” In this manner, one or more spectral components can be dropped from a WDM signal.
In the transmissive state, the reflective device is positioned out of the optical path of the respective spectral component. Thus, spectral components entering switches that are in a substantially transmissive state, such as switch S11, pass through those switches undisturbed and are launched into waveguides, such as waveguide 1551, leading to a second WGR 160. Spectral components entering the WGR 160 are recombined or remultiplexed. Remultiplexed signal 165 appears at the output of WGR 160 and is launched into waveguide 170 and delivered to port 175 of second optical circulator 180. Optical circulator 180 delivers WDM signal 165 to port 185 and into waveguide 190 for transmission to a network node or the like.
One or more spectral components can be added to the original group of spectral components 1051–105N comprising WDM signal 105 (less any dropped spectral components). Such addition is accomplished by delivering the spectral components to be added to port 195 of optical circulator 180. For clarity of description, the addition of only one spectral component 105N+1 is described below. It should be understood, however, that the N switches can add N spectral components, assuming a like number of spectral components are dropped from the original signal.
The one additional spectral component 105N+1 is advanced from port 195 to port 175 and launched into waveguide 170 towards the WGR 160. The WGR 160 delivers the spectral component onto the appropriate one of waveguides 1551–155N as a function of wavelength. Spectral component 105N+1 is assumed to have a wavelength appropriate for occupying the channel vacated by dropped spectral component 1052. As such, spectral component 105N+1 is launched into waveguide 1552 and encounters switch S12.
Recall that switch S12 is in a substantially reflective state to effect the above-described “drop” of spectral component 1052. As such, spectral component 105N+1 is likewise reflected upon entering the switch S12, but towards WGR 160, there to be multiplexed along with other spectral components 105, and 1053–105N into WDM signal 165.
FIG. 2 (prior art) depicts a conventional ROADM 200 similar to ROADM 100 of FIG. 1, like-labeled elements being the same. Unlike ROADM 100, however, ROADM 200 is adapted to equalize the output power of the spectral components on waveguides 15511–155N. To accomplish this, variable-optical attenuators 2051–205N are inserted into waveguides 1551–155N. For a detailed discussion of a ROADM with equalization, see U.S. Pat. No. 6,539,148 to Kim et al., which is incorporated herein by reference.
FIG. 3 (prior art) depicts a second conventional ROADM 300 in which N 2×2 switches S21–S2N replace the 1×1 switches of ROADM 100. When using 2×2 switches S21–S2N, optical circulators 120 and 180 used in conjunction with the 1×1 switches of the first illustrative embodiment are no longer required, as is described below.
ROADM 300 receives a WDM signal 105 via a waveguide 135. A 1×N WGR 140 demultiplexes WDM signal 105 into its constituent spectral components 1051–105N. The N spectral components of WDM signal 105 appear on the outputs of WGR 140 and are carried by waveguides 1421–142N to N 2×2 switches S21–S2N. Each switch S21––S2N can be either a constant-reflectivity device or a variable-reflectivity device. If a variable-reflectivity device is used, the switching function is obtained, i.e., the path of an optical signal traveling therethrough is changed, by a controlled change in reflectivity, such as between substantially transmissive and substantially reflective. If a constant-reflectivity mirror is used, the switching function is obtained by moving the mirror into and out of the path of an optical signal traveling through the switch, again placing the switch in respective substantially reflective or transmissive states. Such movement is actuated, in some embodiments, by a Micro Electro Mechanical Systems (MEMS) based actuator.
The switches may be placed, on an individual basis, in a transmissive state, wherein optical director 305 is substantially “invisible” to a spectral component traveling therethrough. Alternatively, the switches may be placed, again on an individual basis, in a reflective state, wherein optical director 305 reflects a substantial portion of a spectral component incident thereon.
The disposition of each spectral component 1051–105N, i.e., dropping or passing signal to output 165, is controlled by respective associated 2×2 switch S21–S2N. The switches have two inputs and two outputs. First input IN1 of each switch receives one of spectral components 1051–105N delivered to it from one of waveguides 1421–142N. If the switch is in a transmissive state, the one spectral component crosses the switch and is coupled into first output OUT1 for delivery to WGR 160 along appropriate waveguide 1551–155N.
For example, switch S21 is in a transmissive state. Spectral component 1051 delivered to input IN1 of switch S21 via waveguide 1421 crosses the switch, couples to output OUT1 of switch S21 and is launched into waveguide 1551. Thus, spectral components, such as component 1051, entering switches that are in a transmissive state, traverse such switches undisturbed and are launched into waveguides, such as waveguide 1551, leading to a second (N×1) WGR 160. Spectral components entering the WGR 160 are recombined or multiplexed therein. Multiplexed signal 165 appears at the output of WGR 160 and is launched into waveguide 170 for transmission to a network node or the like.
If the switch is in a reflective state, the one spectral component received at input IN1 is coupled into second output OUT2 and launched into waveguide 310i for drop. For example, in the exemplary embodiment of a ROADM 300, switch S22 is in a reflective state. Spectral component 1052 delivered to input IN2 of the switch S22 via waveguide 1422 encounters optical director 305. Upon contact with optical director 305, spectral component 1052 is reflected towards, and coupled with high efficiency into, second output OUT2 of switch S22 and launched into waveguide 3102 for drop.
For switches that are in a reflective state, second input IN2 can be used for adding a spectral component to the WDM signal. The added spectral component is delivered to IN2 via “add” waveguide 315i, and is then coupled into first output OUT1 and launched into waveguide 155i. For example, in switch S22, spectral component 105N+1 is added by delivering it to add waveguide 3152. As spectral component 105N+1 encounters optical director 305, it is reflected towards, and couples with high efficiency into, waveguide 1552. The spectral component is delivered to second WGR 160 and is multiplexed, along with spectral components 1051 and 1053–105N, into output signal 165.
For a more detailed discussion of conventional ROADMs, see U.S. Pat. No. 5,974,207 to Aksyuk et al., which is incorporated herein by reference.
Conventional optical systems are bulky and expensive. (See “Optical MEMS platform for low cost on-chip integration of planar light circuits and optical switching,” by Joel Kubby et al., of Xerox Corporation (2002), which is incorporated herein by reference). Kubby et al. pointed to component integration as one way to significantly reduce prices. To that end, Kubby et al. proposed a Silicon-On-Insulator (SOI) platform for integrating optical, mechanical, and electrical functions. There nevertheless remains a need for small, reliable optical components and switching systems that can be produced with reduced per-channel costs.